How do I append one string to another in Python

Question

I want an efficient way to append one string to another in Python.

var1 = "foo"
var2 = "bar"
var3 = var1 + var2
 

Is there any good built-in method to use?

Answer 1

If you only have one reference to a string and you concatenate another string to the end, CPython now special cases this and tries to extend the string in place.

The end result is that the operation is amortized O(n).

e.g.

s = ""
for i in range(n):
    s+=str(i)
used to be O(n^2), but now it is O(n).